As methods for imparting conductivity to a resin, there have been hitherto proposed a method of adding a conductive filler such as metal oxides, conductive carbon black or the like to the resin.
To obtain preferable conductivity in this method, it is necessary to add a comparatively large amount of the conductive filler to the resin. Thus there have occurred various problems including a problem that the original property of a base resin changes.
Such being the case, investigations have been made to develop a resin material capable of displaying a preferable conductivity, even though the mixing amount of the conductive filler is small.
As disclosed in Japanese Patent Application Laid-Open No. 2005-54095 (patent document 1), there was proposed a conductive resin material containing carbon dispersed in a mixture consisting of not less than two kinds of resin.
Description is made in the patent document 1 that as carbon, it is preferable to use fibrous carbon represented by carbon nanotubes whose diameters are not more than 1 μm and vapor-grown carbon fibers. Description is also made in the patent document 1 that even though the mixing amount of the carbon is as small as 0.01 to 10 mass % of the entire material, the conductive resin material is capable of obtaining preferable conductivity having a volume resistivity not more than 1010 Ω·cm.
But the conductive resin material described in the patent document 1 has a problem that when the conductive resin material containing a small amount of the conductive filler is molded into a sheet-like material such as a film, a variation in the in-plane electric resistance of the sheet-like material is large or the flexing resistance of the sheet is poor in dependence on a dispersion state of the conductive filler.    Patent document 1: Japanese Patent Application Laid-Open No. 2005-54095